Glad someone jumped in here and corrected the idea that the OBD2 setup somehow requires a bunch of other modules to run. The only thing more complex about the OBD2 stuff is more wires to remove from the harness and maybe a little more research to figure what the PCM triggers (e.g. does the fuel pump power off the ASD relay, or does the PCM trigger a separate relay).
The only reason I can think you would need stuff like the BCM and ABS module would be if you wanted the PCM to control the cruise control or something, and even then I think it is just the ABS module it needs. If you wanted to run the Ram IP, you would need the BCM since it controls many of the idiot lights. But none of that is needed to make the motor run.
There might be some complications with sensors as the PCM needs to see things like water temp which might or might not be the same signal as the gauge needs. The OBD1 stuff still used mechanical gauges so that is different.
The way I see it is, if you want to run a stock setup without mods, and want maximum simplicity, run an OBD1 system. If you want to tune it (even someday), need (want?) to run an RE trans, and aren't afraid of reading a wiring diagram, no reason not to run the OBD2 system.
I have projects where I am using both. My '91 Dakota is getting a 5.9 swapped in next summer using a '95 PCM for van and a '92 Dakota harness. I have to splice in a third wire for the speed sensor and repin/rewire the IP plugs so I can use a non-mechanical speedometer, and there are a few other tricks to it, but nothing huge. Also in the works is a Hughes EFI intake swap onto my Duster using a 2001 PCM with a '98 Ram harness I cut down and repinned for the PCM along with a custom PDC. That one might end up using a BCM and ABS module, but only because I have wild ideas of using teh built in security feature for the Ram, and the OE cruise control. Granted, neither system is actually on a vehicle and running, but I see no reason they shouldn't.
